Laminates comprising one or more layers of dielectric material, such as polyimide and one or more layers of metallic substrate material may be used for a variety of applications. For example, polyimide coated metal foils, due to the flexibility and outstanding mechanical, thermal and electrical properties of polyimides, can be used for printed electrical circuits. This is because the laminates are frequently exposed to high temperatures during further processing, for example, during soldering or drilling. The laminates also have to satisfy stringent requirements in regard to their electrical and mechanical properties.
Laminates comprising only one substrate layer of metal or metal alloy and a layer of polyimide, so called single clads, may be used for printed electrical circuits. The same applies to multilayer laminates, so called multi-clads or multilayer circuits, which comprise several metal layers and/or several polyimide layers.
The metal layers are usually etched by methods well known in the art to form conductive paths on the polyimide layer.
Especially in the case of multi-clads, it is necessary to form small vias or other holes of various sizes through the polyimide and adhesive layers, or through the whole clad. This is necessary in order to electrically connect the conductive metal paths disposed within different layers, or for other purposes such as, for example, forming conductive bridges, inserting electronic components, and the like. Flexible circuits and tape automated bonding parts are examples where laminates using adhesives and techniques of the present invention are very useful.
Although in a number of occasions, the vias or holes may be punched, mechanically drilled, or laser drilled, it is desirable in many other occasions to use chemical etching techniques for this purpose. In the case where the method of opening the vias or holes is chemical etching, it is obviously necessary to use etchable materials as a dielectric film and as an adhesive.
Laminates containing polyimides and metal substrates are well-known in the art. Usually the polyimide layers are bonded to the metal substrate by a conventional adhesive. For example, U.S. Pat. No. 3,900,662, U.S. Pat. No. 3,822,175, and U.S. Pat. No. 3,728,150 disclose bonding of polyimide to metal using an acrylate-based adhesive. However, it has been found that when conventional adhesives, such as acrylates for example, are used to bond the polyimide to the metal, the resulting laminates do not exhibit entirely satisfactory properties which meet the stringent demands often imposed. In addition, they do not possess chemical etching characteristics.
Thus, adhesives initially based on epoxy chemistry and later based on imide chemistry became of higher preference, but still the problem of inability to etch these adhesives remained mostly unresolved. In order to avoid the use of adhesives at all, multilayer laminates have been proposed in which the polyimide is bonded directly to metal, i.e., without a layer of adhesive. Thus, British Patent 2,101,526 discloses the bonding of a polyimide derived from biphenyltetracarboxylic dianhydride directly to metal foil by applying heat and pressure. The whole polyimide layer of this laminate, however, is subject to inferior thermal stability as compared to laminates made from layers of conventional polyimides. Further, the selection of polyimides to be used in such laminates is limited.
Currently available acrylic and epoxy based adhesives are non-etchable chemically, while long molecular weight linear polyimides may be often chemically etchable. However, the linear polyimides, in order to have adequate physical properties they must be of high molecular weight, which in turn brings about a serious disadvantage. The solution viscosity of these polyimides is excessively high at reasonable solids content levels. This renders them impractical as adhesives, since a very large number of layers have to be applied before reaching a reasonable thickness.
Thus, according to this invention, cross-linkable or extendable oligomers are preferred, which contain in their backbone phenolic ester bonds, defined as bonds which are the reaction product of a carboxylic acid with a phenolic hydroxyl group. The oligomers may be easily selected to have low viscosity, due to their low molecular weight. The molecular weight is then increased in the laminate by cross-linking or extension as described in more detail later, and the chemical etchability is brought about by the incorporated phenolic ester groups.
U.S. Pat. No. 4,517,321 (H. C. Gardner, et al.) discloses resins with improved tensile properties and compressive strength by using as curing agents such diamines as aromatic dietherdiamines, among which a diester diamine made from bisphenol A by esterification with meta-aminobenzoic acid is also disclosed. However, no mention or suggestion is made regarding the difference, which is of utmost importance to the present invention, of this particular diester diamine when compared to the plethora of the rest of the diether diamines. This difference, which is immaterial for the purposes of the invention disclosed in U.S. Pat. No. 4,517,321, is the fact that a diester diamine resulting from a carboxylic acid, such as, for example, aminobenzoic acid and a phenolic hydroxyl bearing compound, such as bisphenol A, provides chemical etchability to the cured product. The same comments apply to U.S. Pat. No. 4,579,885 (Linda A. Domeier, et al.), which also discloses similar compositions.
U.S. Pat. No. 4,898,971 (W. E. Slack) discloses liquid isocyanate prepolymers made from aromatic diisocyanates and lengthy, flexible diamines some of which may use ester functions to link flexible segments. However, again nothing is mentioned, suggested or implied regarding the etchability-imparting difference between the aforementioned special ester group and the rest of the compositions.
U.S. Pat. No. 4,851,495 (Sheppard et al.) discloses polyetherimide oligomers having cross-linking and end cap moieties, which provide improved solvent-resistance to cured composites. It also discloses blends generally comprising substantially equimolar amounts of the oligomers and a comparable, compatible, non-cross-linking, etherimide polymer of substantially the same backbone. Sheppard utilizes all aromatic moieties with ether (--O--) or thioether (--S--) linkages as flexibilizing functions. To achieve any melt flow away from cure temperatures, m in his formula must be kept no more than 0 or 1. However, this makes the cured resin brittle and suitable only for rigid laminates and/or composites. Even for those applications, brittleness is probably the reason for resorting to blends with reactive plasticizers.
U.S. Pat. No. 4,801,682 (Scola) discloses high temperature polyimides, which are typically the copolymerization product of about 3 mole % to about 42 mole % nadic esters; about 39 mole % to about 49 mole % diamine; and about 17 mole % to about 48 mole % 4,4', 9 (2,2,2-trifluoro-1-phenyletheridene)-biphthalic tetracarboxylic acid dialkylester. This chemistry deals with structural composites, where evolution of volatiles is not important. There is an abundance of volatiles because this chemistry involves partial esters of di- and tetracarboxylic acids with lower alcohols, which must be liberated during cure.
U.S. Pat. No. 4,711,964 (Tan et al.) discloses bisbenzocyclobutene aromatic imide oligomers. This chemistry also involves structural composites, not suitable for adhesives. Benzocyclobutene end groups may be cured by Diels-Alder conditions requiring high temperatures, and lengthy times, as well as presence of dienenophiles such as commercial bismaleimides, generally leading to brittle resins.
U.S. Pat. No. 4,528,373 (D'Alelio et al.) discloses unsaturated vinylacetylene-terminated polyimides and processes for their preparation. This invention involves high molecular weight polymers terminated in acetylenic functions requiring high post cure temperatures. The cure temperatures may be lowered by mixing in free radical initiators, which however, are inevitably incorporated in the resin with unknown impact on properties.
U.S. Pat. No. 4,064,192 (Bargain) discloses heat-stable resins having good mechanical and electrical properties combined with chemical inertness at temperatures of 200 to 300.degree. C., which resins are resins of a three-dimensional polyimide which is obtained by reacting, at between 50.degree. C. and 350.degree. C., a bisimide of the general formula: ##STR1## in which Y denotes H, CH.sub.3 or Cl, and A represents a divalent organic radical possessing at least two carbon atoms, a polyamine of the general formula: EQU R(NH.sub.2)x
in which x represents an integer at least equal to 2 and R denotes an organic radical of valency x, and an alazine of the general formula: EQU G--CH.dbd.N--N.dbd.CH--G
in which G represents a monovalent aromatic radical, and a polymerizable monomer other than a bisimide, containing at least one polymerizable vinyl, maleic, allyl or acrylic --CH.dbd.C&lt;group in amounts such that if N.sub.1 represents the number of moles of bisimide employed, N.sub.2 represents the number of moles of polyamine employed, and N.sub.3 represents the number of moles of alazine employed, the ratio ##EQU1## is at least 1.3, x being defined as above.
U.S. Pat. Nos. 4,814,357 and 4,816,493 (Indyke) disclose described flexible polyimide foams having enhanced compression fatigue life and softness for use in the manufacture of seat cushions and methods for the production of such foams and precursors therefor. These foams are produced from novel polyimides prepared by reaction of an organic tetracarboxylic acid or derivative thereof, preferably an ester with (a) about 1 to about 50 mole percent of a diester of (i) a primary amino-substituted aromatic carboxylic acid, and (ii) a polymethylene glycol, and (b) at least one aromatic or heterocyclic primary diamine. Foams can be produced having (a) a fatigue life as determined by ASTM test procedure D3574-81 using foam specimens from three to five inches in thickness of at least 15,000 cycles, or (b) an indentation force deflection as determined by ASTM test procedure D3574-81 on foam specimens of one-inch thickness of less than 40 pounds of force at 25% deflection and less than 180 pounds of force at 65% deflection, or both of (a) and (b).